1. Field of the Invention
The invention relates to a process for producing metal/metal foam composite components, in particular for producing shaped metal parts from light metal materials which have a reduced weight compared to conventionally produced shaped parts. The invention also relates to shaped parts produced using this process and to their use in light metal structures.
2. Description of the Related Art
Reducing the weight of shaped metal parts, for example for applications in automotive engineering, aircraft construction or other technologically highly demanding application areas is of considerable economic but also ecological importance. As well as the known use of light metals, foamed metallic materials are also receiving increasing attention. These materials are distinguished by a lightweight structure, a high rigidity and compressive strength, good damping properties, etc., and there are known processes for producing them.
It is known to produce components from foamed metallic materials. By way of example, cast cores of aluminum foam are surrounded with aluminum material by casting or are inserted as shaped parts into a component. The sheath and core or shaped part are produced separately and are then joined to one another. In addition to the high manufacturing outlay, this also leads to a low manufacturing quality. The basis of foamable semifinished aluminum products is atomized aluminum powder to which a blowing agent is added. By way of example, according to DE 197 44 300 A1, a body which has been pressed from a powder mixture is heated, in a heatable, closed vessel, to temperatures which are higher than the decomposition temperature of the blowing agent and/or the melting temperature of the metal.
In this process, the powder is compressed and the shaped part produced in this way is inserted into the area of a component which is to be filled by foaming, and is foamed by heating to up to 650° C. In the process, the sheath may be subject to unacceptable deformation, or the foaming operation may take place nonuniformly. Production of foams by sintering of metallic hollow spheres or infiltration of metal melts into cores or filler bodies, which are removed after solidification of the melt, is also possible.
According to a process described in JP 03017236 AA, metallic articles with cavities are produced by dissolving gases in a metal melt and initiating the foaming operation by suddenly reducing the pressure. The foam is stabilized by cooling of the melt.
According to the teaching given in JP 09241780 AA, metallic foam is obtained with the controlled release of blowing gases as a result of a metal initially being melted at temperatures which lie below the decomposition temperature of the blowing agent used. Subsequent dispersion of the blowing agent in the molten metal and heating of the matrix to above the temperature which is then required to release blowing gases leads to a metal foam being formed.
The casting of metal parts with lost foam is already known in accordance with EP 0 461 052 B1. WO 92/21457 A1 describes the production of aluminum foam in such a manner that gas is blown in below the surface of a molten metal, abrasives being used as stabilizers.
W. Thiele: Fullstoffhaltiger Aluminumschwamm—ein kompressibler Gusswerkstoff zur Absorption von Sto8energie, [Fillercontaining aluminum sponge—a compressible cast material for absorption of impact energy], in: Metall, 28, 1974, Vol. 1, pp. 39 to 42, describes the production of foamed aluminum. The desired cavities are predetermined in terms of size, shape and position in the form of a loose bed of readily compressible, inorganic light materials, such as for example expanded clay minerals, expanded clay, glass foam beads or hollow corundum beads, etc. The bed of light material is introduced into a die. The spaces which remain in the bed are filled with metal. The aluminum sponge obtained in this way has relatively poor mechanical qualities and contains the material of the bed.
DE 11 64 103 B describes a process for producing metal foam bodies. In this process, a solid material which, when heated, decomposes to form gases, is mixed with a molten metal in such a manner that the solid material is wetted by the metal. By way of example, pulverulent titanium hydride is added to a molten alloy of aluminum and magnesium at a temperature of 600° C. The closed foam formed in this way is then cast into a die, where it can cool and solidify. In this case too, it is clearly not a closed system, but rather an open system which is used.
GB 892934 relates to the production of complex structures with a foamed metal core and a closed, nonporous surface.
DE 198 32 794 C1 describes a process for producing a hollow profiled section which is filled with metal foam. This process comprises the steps of extruding the hollow profiled section from a sheathing material using an extruder which has an extrusion die with a die part and a mandrel, supplying the metal foam from a foam material to the hollow profiled section through a feed duct, which is formed in the mandrel
JP Patent Abstracts of Japan 07145435 A describes the production of foamed metal wires. Molten aluminum is foamed in a furnace with the aid of a blowing agent and is fed to a continuously operating casting device. The molten aluminum in the foamed state is cooled between a pair of upper and lower conveyor belts in order to obtain an endless strand. This is cut in a predetermined way to form the foamed aluminum wires. Alternatively, the foamed aluminum wire or strand may be shaped by drawing the foamed, molten aluminum between a wire with a groove and a conveyor belt. Therefore, the molten aluminum wire is obtained by rolling or drawing.
EP 0 666 784 B1 describes a process for the shape casting of a metal foam which is stabilized by means of particles, in particular an aluminum alloy, by heating a composite of a metal matrix and finely divided solid stabilizer particles above the solidus temperature of the metal matrix and discharging gas bubbles into the molten metal composite below the surface thereof to thereby form a stabilized liquid foam on the surface of the molten metal composite. The characterizing feature is shape casting of the metal foam by the stabilized, liquid foam being pressed into a die, using a pressure which is just sufficient for the liquid foam to adopt the shape of the die, without the cells of the foam being significantly compressed, and then cooling and solidifying the foam, in order to obtain a shaped object. The foam is in this case pressed into the die by means of a moveable plate. A first moveable plate presses the liquid foam into the die, and a smooth surface is formed on the shaped foam object. A second moveable plate is pressed into the foam inside the die, in order to form smooth inner surfaces on the foam object. However, the shaping may also take place by means of rollers.
A further process for making castings from metal foam is described in EP 0 804 982 A2. In this case, the foaming takes place in a heatable chamber outside a die, the volume of the powder metallurgy starting material introduced into the chamber for the metal foam, in its phase in which it has been foamed with the entire foaming capacity, substantially corresponding to the volume of a filled die. All the metal foam in the chamber is pressed into the die, in which foaming with the remaining foaming capacity is continued until the die has been completely filled. The die is a sand or ceramic die, the metal foam is inserted into the chamber as a semifinished product and is only pressed into the die, for example by means of a piston, after the initial foaming. When the foam is being pressed into the die, it is sheared. The die is not filled with a foam with a structure which is deliberately inhomogeneous.
DE 195 01 508 C1 discloses a process for producing a hollow profiled section of reduced weight and increased rigidity, for example a component for the chassis of a motor vehicle. It comprises die-cast aluminum, in the cavities of which there is a core of aluminum foam. The integrated foam core is produced by powder metallurgy and is then fixed to the inner wall of a casting die and surrounded with metal by pressure die-casting.
DE 297 23 749 U1 discloses a wheel for a motor vehicle which comprises at least one metallic foam core which is exposed toward the inner side of the wheel and has a cast wall toward the outer side of the wheel. The foamed core comprising aluminum foam is inserted into a permanent die in order to cast the wheel and is positioned in such a way that, during casting, the outer cast skin is formed between the permanent die and the foamed core.
DE 195 02 307 A1 describes a deformation element, in the housing of which a filling comprising an aluminum foam is provided as energy-absorbing means. The housing may consist of metal or plastic. The filling body is a simple insert part without any material-to-material bonding to the housing.
The dissolving or blowing of blowing gases into metal melts is not suitable for the production of near net shape components, since a system comprising melt with occluded gas bubbles is not stable for a sufficient time for it to be processed in shaping dies.